1. Field of the Invention
This invention relates to a system consisting of a microphone and a preamplifier, wherein the microphone comprises a housing having therein an opening to the surroundings, a diaphragm and a backplate, while the amplifier is coupled to the system consisting of the diaphragm and the backplate and has a field effect transistor as input element.
2. Description of the Prior Art
The microphone is more particularly the microphone of a hearing aid. Such microphones and the associated preamplifiers (also hereinafter referred to as amplifiers) have been miniaturized further and further over the last years, so that, for instance, they can be accommodated inside, instead of outside, the ear, or even in the auditory duct. Accordingly as a microphone becomes smaller, its capacitance is generally lower. Since the noise produced by an amplifier is inversely proportional to the capacitance at the input thereof, such miniaturization of the microphone has as a drawback that the noise thereof increases more and more and has actually become dominant with respect to the noise of the amplifier, which used to be determinative. In the microphone, in the partition between the so-called front volume, which communicates with the surroundings via an opening sometimes referred to as the “snout”, and the back volume, closed off with respect to the surroundings, a pressure equalization opening is present, which ensures that the back volume is not going to function as a barometer, thereby adversely affecting the operation of the microphone. The size of this pressure equalization opening also influences the noise behavior, because, in acoustic terms, it constitutes a resistance, and must therefore be dimensioned very accurately, with tolerances in the order of micrometers, which is a great technical problem.
For influencing the frequency behavior of a miniature microphone and improving the noise behavior, further, in the snout, often an element, such as a gauze or a drop of glue, is provided to attenuate high frequencies. Such elements can become soiled, for instance by earwax, so that not only the snout may become clogged, but also the frequency behavior is uncontrollably influenced.
Finally, a miniature microphone has a frequency response with a resonance peak in the audible high-frequency range, which also affects the noise behavior adversely. If the resonance peak is placed outside the audible frequency range, this, in turn, has yet other adverse consequences for the noise behavior.